(1)/(3)x(15X-12Y)-(1)/(2)(5x-4Y)
I can't tell whether x is a variable, or a multiplier, or both. I shall assume you mean
1/3 (15x-12y) - 1/2 (5x-4y)
If so, then that simplifies as follows, using the distributive property
5x-4y - 5/2 x + 2y
= 5/2 x - 2y